Among brushless motors such as switched reluctance motors the demand for which has increased recently because they are inexpensive and simple in structure are ones that incorporate an encoder for outputting a pulse signal in synchronism with the rotation of the rotor and in which the rotor is rotated by sequentially switching the current supply phase by counting pulses of the output signal of the encoder and detecting the rotation position of the rotor on the basis of the count.
First, in this type of motor having an encoder, the rotation amount (i.e., a rotation angle) from a start position of the rotor can merely be detected on the basis of the count, after starting, of an output signal of the encoder. Therefore, the motor cannot be driven normally unless a corresponding relationship between the rotor rotation position and the current supply phase is obtained by detecting the absolute rotation position of the rotor by a certain method after the application of power.
In this connection, for example, JP-A-2000-69779 discloses a switched reluctance motor in which current supply is simultaneously effected for two phases at the beginning of a starting period and a current supply phase is determined after a lapse of a prescribed time by using, as a reference, a rotation position of the rotor at that time. However, mere simultaneous two-phase current supply at the beginning of a starting period not always causes the rotor to rotate to a position corresponding to the two current supply phases; there may occur a case that a corresponding relationship between the rotor rotation position and the current supply phase is not obtained. Another mode is proposed in which current supply is effected for one phase first and for two phases simultaneously.
An unstable region that will be caused by two-phase current supply is eliminated by effecting one-phase current supply first. The rotor is rotated to a single stable point by later two-phase current supply and a reference position of the rotor is thereby learned. However, producing lower torque than two-phase current supply does, one-phase current supply may not be sufficient for the rotor to rotate to a corresponding position. Therefore, merely effecting one-phase current supply first may not eliminate an unstable region that will be caused by two-phase current supply, and a reference position of the rotor may still be learned erroneously.
Second, where a position switching control is performed with a motor having an encoder, if the rotation direction of the rotor is revered due to some inexorability while the rotor is rotated to a target position on the basis of the count of the encoder, the count changing direction of the encoder is also reversed. As a result, the current supply phase switching order that is determined on the basis of the count of the encoder is also reversed to generate torque that drives the rotor in the reverse direction. Once the rotor starts to rotate in the reverse direction, the reverse rotation is accelerated rather than suppressed. This raises a problem that long time is needed to return the rotation direction from the reverse direction to the normal direction; the rotor may reach a target position with a long delay or, in the worst case, it may become impossible to stop the reverse rotation, that is, the motor may become uncontrollable.
Third, where the rotor is rotated to a target position by a feedback control, the following control is performed. Every time anew target position is set, a feedback control is performed in which the rotor is rotated toward the target position by switching the current supply phase sequentially on the basis of the count of the encoder. When the count of the encoder has reached a target count that is set as corresponding to the target position, it is determined that the rotor has reached the target position, whereupon the feedback control is finished and the rotor is stopped at the target position. In this case, the rotor can be kept at the target position by electromagnetic force by continuing to energize the winding of the phase corresponding to the target position after completion of the feedback control. However, in this configuration, if the rotor is stopped for a long time, the winding of the same phase continues to be energized for a ling time and hence may overheat and burn. To prevent the winding from overheating or burning, the winding is not energized while the rotor is stopped.
However, where the rotor is not energized while it is stopped, there is no electromagnetic force for keeping the rotor at a target position (i.e., a position when the rotor stopped) and hence the rotor may deviate from the target position. One countermeasure is to provide a mechanical stopping and holding mechanism for keeping the rotor at a target position by spring force or the like. However, even with this measure, the rotor may still deviate from the target position due to play amount in the stopping and holding mechanism, its variations in manufacture, or the like. If the position of the rotor deviates in a period when it should be stopped, a feedback control is restarted from a current supply phase that is different from A-phase for which current supply should be effected first. In this case, the rotor may not be rotated normally to a target position; for example, a loss of synchronization occurs at the start of the feedback control to cause a failure of starting or the rotor is rotated away from the target position.
If the rotor deviates only slightly while it is stopped and the position of the rotor stays within a range corresponding to a count of the encoder with which the preceding feedback control finished, current supply is effected first for A-phase with which the preceding feedback control finished. However, a feedback control is performed by using, as a reference, a position of the rotor for current supply. Therefore, even if the deviation of the rotor is so small that its position stays within a range corresponding to a count of the encoder, a loss of synchronization may still occur at the start of a feedback control to cause a failure of starting if the feedback control is started after current supply is effected for A-phase for which current supply should be effected first and before the rotor is moved to a position for current supply and kept there.
Fourth, the rotation amount (i.e., rotation angle) of the motor is converted via a rotation transmission system into the manipulated variable for a control object (a position switching device). However, play amount due to backlash, clearances, etc. exists between constituent parts of the rotation transmission system. Therefore, even if the rotation amount of the motor can be controlled correctly on the basis of the count of the encoder, an error corresponding to the play amount in the rotation transmission system occurs in the manipulated variable for the control object and hence the manipulated variable for the control object cannot be controlled accurately.
Fifth, in rotating the rotor to a target position, the current supply phase switching is performed on the basis of the count of the encoder. To generate torque for rotating the rotor, the phase of the current supply phase need to lead the rotation phase of the rotor. As the rotation speed of the rotor increases after a start of driving, the variation rate of the count of the encoder increases and the current supply phase switching becomes faster. However, torque is generated actually with a delay corresponding to the inductance of the winding of a current supply phase from a start of current supply of the winding. Therefore, if the rotation speed of the rotor is too high, the rotor rotates by a considerable angle from a start of current supply of the current supply phase winding to actual torque generation, that is, the generation of torque of the current supply phase delays from the actual rotation phase of the rotor. In this state, the driving torque decreases and the rotation speed of the rotor lowers. A requirement of increase in position switching rate (i.e., increase in the rotation speed of the rotor) cannot be satisfied.
One countermeasure against this problem would be setting the phase lead of the current supply phase to a large value in advance. However, if the phase lead of the current supply phase is large at a start of driving (i.e., at starting), the starting torque becomes low and the starting of the motor becomes unstable or results in a failure. Further, if the phase lead of the current supply phase is set large and the rotation speed of the rotor is thereby increased, the rotor tends to pass a target position due to inertia at an end of driving (occurrence of an overshoot), that is, it is difficult to stop the rotor at the target position correctly. Another measure for increasing the stability of the position switching control would be effecting current supply in such a manner that the current supply phase is selected so that rotor is temporarily stopped and kept there at a start of driving, at an end of the driving, and when the target position is changed (or the rotation direction is reversed). However, if this is done by one-phase current supply that produces low position-keeping torque, the rotor vibrates and cannot be stopped completely at each position.
Sixth, the motor is controlled by a microcomputer and the microcomputer may be reset for a certain reason (e.g., a short power break) while controlling the motor. If the microcomputer is reset, target position data after the resetting become different from those before the resetting because a RAM for storing target position data is also reset. As such, resetting of the microcomputer may cause a problem that a control object is switched contrary to the intention so as to be directed to an unintended position.
Seventh, where a motor feedback control according to JP-A-2001-271917 is employed, the current supply phase is switched in synchronism with output of pulses of a pulse signal of the encoder. Therefore, if the rotation of the rotor is stopped for a certain reason during a feedback control and the output of the pulse signal from the encoder is also stopped, the current supply phase switching can no longer be performed. This means a problem that the rotor cannot be rotated to a target position.